Mount Tamalpais is the highest peak in the Marin Hills, which are part of the Northern California Coast Ranges. The elevation at the East Peak, its highest point, is 2,574 feet (785 m). The West Peak, the mountain's second highest peak, where a radar dome currently stands, is at about 2,563 feet (781 m).[4] It stood over 2,600 feet (792 m) before the summit was flattened for the radar dome construction. The mountain is clearly visible from the city of San Francisco and the East Bay region.

In 2004 it was suggested by a team of Penn State geoscientists that a blind thrust fault, like the one that caused the infamous Northridge earthquake, lies beneath Mount Tamalpais. This idea was partly based on the steepness of Mount Tamalpais and of nearby Bolinas Ridge, such steepness on the visible surface often being the result of blind thrust faults. Another reason for the suggestion was that the San Andreas Fault creeps more slowly south of Mount Tamalpais than it does in its sections north of Mount Tamalpais and in the Olema Valley, and that the existence of a blind thrust fault may explain the different creeping velocities. If a blind thrust fault does exist under Mount Tamalpais, and if it ruptures, it could be potentially devastating to the North Bay, San Francisco, and any other nearby locale resting on unstable earth and loose fill.[5]

Major Mount Tamalpais rockforms include serpentine, particularly evident in outcroppings near the summit and on the north side. A number of serpentine endemic plants grow in the serpentine soils in this part of the mountain.

Since the steep slopes of Mount Tamalpais force out moisture from passing storms and/or fog, the mountain supports several year-round streams like Redwood Creek on the southern face of the mountain down into Muir Woods. The steep southeastern slopes of Mount Tamalpais drain to Arroyo Corte Madera del Presidio, which in turn discharges to Richardson Bay.

With its height, various faces, and proximity to the ocean and bay, the mountain contains many microclimates, ranging from cool and foggy in lower ocean-facing valleys with their redwood forests, to hot and dry on the manzanita slopes, cool and breezy at the summit, and shady on the heavily Douglas-fir-forested north slopes near Alpine Lake.

Annual precipitation[6] around Mount Tamalpais varies greatly from around 27.5–31.5 inches (700–800 mm) in the drier, eastern foothills to about 59 inches (1,500 mm) near the Bolinas Ridge, close to the Pacific Ocean. Both Mount Tamalpais and the Bolinas Ridge force moisture out of the air efficiently, since the air is cooled rapidly as it ascends the steep mountain faces and thus Mount Tamalpais's western part is heavily forested with tall redwoods and Douglas firs. The same fact holds for the steep, south-facing bowl canyon that Muir Woods is located in, with precipitation in Redwood Canyon at around 39.4–47.2 inches (1,000–1,200 mm).[7]

As in San Francisco, most of the annual precipitation falls during the winter months. During cold, wet winter storms, the mountain also regularly gets some snowfall, sometimes as much as 6 inches (15 cm) overnight, as observed in February 2001, March 2006, and February 2011.[8] The region sometimes gets hit with strong Pacific storms that may topple trees, and bring hurricane force winds to exposed, barren areas like the Bolinas Ridge and the summit of Mount Tamalpais. In summer, the area gets almost no precipitation, except for fog drip[9] that occurs in Muir Woods, the Bolinas Ridge and the western end of Mount Tamalpais, where summer fog and oceanic breezes are more prevalent. In contrast, the eastern foothills, sheltered from the oceanic breezes and fog, are drier, since the foothills force little moisture out of the air. This leads to the fact that the eastern slopes contain only oak, pine, shrub, woodland, and sparse Douglas-fir forest.

Temperatures on top of Mount Tamalpais are generally somewhat cooler than places next to the San Francisco Bay or the ocean due to elevation. In summer, however, the top of Mount Tamalpais may actually be warmer than the middle, foggy elevations due to a thermal inversion. The summer fog and breezes make locations on Mount Tamalpais, closer to the ocean, cooler than the blazing hot interior valleys.

Open Douglas-fir/oak woodland/grassland to the west transitioning to a mostly dense Douglas-fir forest to the east

Hardwood woodland types are generally subtypes of California oak woodland, including oak-bay-madrone forest, oak woodland, and oak savannah. Oak-bay-madrone forests are found in areas with moderate moisture and particularly favor north-facing slopes. They are dominated by one or more of three hardwood tree species – Coast Live Oak, California bay, and madrone. Coast live oak tends to be dominant in somewhat drier areas, while bay is more dominant in shadier, moister areas; madrone is abundant in certain soil types in both moist and dry spots. Oak woodlands are a more open-canopy forest dominated by coast live oak, while oak savannah has a completely open canopy and represents a mixture of coast live oak woodland and grassland.[10]

The great diversity of microclimates on Mount Tamalpais ensures a wide variety of plant communities as well. Plant communities on the mountain include various types of hardwood and coniferous forests, coastal scrub, chaparral, grassland, and wetland vegetation. Wholly or partially coniferous forest types are found in the moistest areas of Mount Tamalpais. Coast redwood forests are restricted to areas where the particular ecological needs of redwood are met: areas characterized by high overall moisture, low elevations below the fog line, and deep soils. Muir Woods is the most extensive and best-known redwood forest of the Mount Tamalpais area. Mixed evergreen forests of various combinations of tanoak, madrone, coast and canyon live oak, and Douglas-fir are found in moist areas on middle to high elevations on the mountain. Very moist areas of mixed evergreen forest may also include bay, redwood, and California torreya. Areas in which mixed evergreen forests are predominant include areas of Fairfax-Bolinas Road and Ridgecrest Boulevard and around Alpine Lake.[10]

A stand of Douglas-firs near the summit

Various kinds of scrub communities are also widespread. Low-elevation areas below the fog line with relatively low overall rainfall or thin soils are often the site of a northern coastal scrub community characterized by coastal sage-coyote brush association, with lesser amounts of poison-oak, bush monkeyflower, California blackberry, western bracken fern, and various species of grasses and forbs. Chaparral is predominant in areas characterized by thin, rocky soils and little moisture. Two main types of chaparral are found the mountain chamise chaparral and manzanita chaparral. Chamise is dominant in the hottest, most xeric areas of the mountain, particularly on south- and west-facing slopes, while manzanita is dominant in other xeric areas, particularly on east-facing slopes and forest borders. Areas of mixed chamise-manzanita chaparral occur in relatively more mesic areas; Ceanothus and dwarfed interior live oak may also predominate on such sites. Areas in which various kinds of chaparral communities are dominant include areas along Old Railroad Grade.[10]

Wetland vegetation types found on Mount Tamalpais include coastal riparian forests, wet meadows, and some marsh areas. Coastal riparian forest is predominant along the valley streams of Mount Tamalpais. Red and white alder (Alnus rubra and Alnus rhombifolia) and arroyo and yellow willow (Salix lasiolepis and S. lasiandra) are dominant in these types of woodland, with bigleaf maple (Acer macrophyllum), box-elder (Acer negundo ssp. californicum), and California bay also being common.[10] Wet meadows are present in several high-elevation spots on the mountain,[11] while High Marsh represents a rare example of a marsh community on the mountain.

Serpentine soils have a high rate of endemism and are the site of several unique subtypes of the above plant communities. Serpentine grasslands are some of the few grasslands in which native perennial grasses are still relatively dominant. Serpentine chaparral forms a unique plant community, dominated by dwarfed leather oak (Quercus durata), Jepson ceanothus (Ceanothus jepsonii), Tamalpais manzanita (Arctostaphylos montana), and Sargent cypress (Cupressus sargentii). On the upper slopes of the mountain, small groves of Sargent cypress trees up to 50 feet (15 m) tall can be found in serpentine areas.[10]

Several species of endemic plants are found only on serpentine soils; these species may be widespread, but only occur on serpentine soils, or the may be more restricted, only growing in a few other places besides Mount Tamalpais, or may even be restricted just to Mount Tamalpais. The Mount Tamalpais thistle (Cirsium hydrophilum var. vaseyi), for example, is a rare variety of thistle known only from the serpentine seeps of the mountain.[12] The Mount Tamalpais jewelflower (Streptanthus batrachopus) is also limited to the area.

Mount Tamalpais provides one of the last remaining wildlife refuges in the Bay Area. Urbanization has invaded wildlife habitat, forcing many fauna in southern Marin County to retreat up onto Mount Tamalpais, Muir Woods, and the Bolinas Ridge. A wide variety of avifauna, amphibians, arthropods and mammals are found on Mount Tamalpais, including a number of rare and endangered species. Nonetheless, Mt. Tamalpais and the neighboring Golden Gate Recreation Area together encompass over 115 square miles (298 square kilometers) of land, forming one of the largest preserved parklands located near a U.S. urban center.

The name Tamalpais was first recorded in 1845. It comes from the Coast Miwok name for this mountain, támal pájiṣ, literally "west hill".[13] Various different folk etymologies also exist, but they are unsubstantiated. One holds that it comes from the Spanish Tamal país, meaning "Tamal country," Tamal being the name that the Spanish missionaries gave to the Coast Miwok people. Another holds that the name is the Coast Miwok word for "sleeping maiden" and is taken from a "Legend of the Sleeping Maiden."[14][15][16] However, this legend actually has no basis in Coast Miwok myth and is instead a piece of Victorian-era apocrypha.[16][17][18] Another suggests a tie to the Asian origins of the Miwoks, where "pais" means place and "tamal" is a tribe in Siberia.[19]

The Coast Miwok are said to have believed that an evil witch dwelled at the top of Mount Tamalpais and therefore never set foot on the peak.[16] However, it has been said the Miwoks only said this in order to keep settlers off the sacred mountain.[citation needed]

Mount Tamalpais is a popular hiking, picnicking, mountain and road cycling, horseback riding, and hang-gliding destination for residents of the San Francisco Bay Area, with over 100 miles (160 km) of trails and fire roads. With numerous trailheads, a well-networked trail and road system, and hikes of greatly varying length and difficulty, the mountain offers a compelling range of attractions. Marin Municipal Water District maintains several reservoirs on the north slopes of Mount Tamalpais, including Alpine Lake, Kent Lake, Bon Tempe Lake, Phoenix Lake, and Lake Lagunitas.

The western slopes of the mountain descend to the Pacific Ocean at Stinson Beach. The annual Dipsea Race traverses the mountain from Mill Valley to Stinson Beach. Though backpack camping isn't allowed, walk-in camping exists at the Pan Toll ranger station and Bootjack Campground. The historic Bootjack Campground was originally established in the early 1920s by the Tamalpais Conservation Corps , and the Civilian Conservation Corps further refined the campground facilities in the 1930s. Showing substantial signs of overuse, the campground was closed by State Parks in 1969. The Bootjack Campground and Trail were re-opened on Thursday June 5, 2014.[24]

Trailhead parking within Mt. Tamalpais State Park is available generally with a self-service fee ($8). The scenic Ridgecrest Blvd. running along the ridgeline between the Rock Spring trailhead and Fairfax-Bolinas Road, with panoramic views of the summit, Pacific, San Francisco, Bolinas, and Point Reyes, is featured in many automobile and other advertisements, as well as being the local hang-gliding launch point. Mount Tam is also home to the Edgewood Botanic Garden and to the Sidney B. Cushing Amphitheatre where Broadway musical productions are performed every year by the Mountain Play Association. Monthly astronomy viewings and lectures are held at Rock Spring and Mountain Theater April through October.

Since the Mount Tamalpais area contains large expanses of undeveloped, natural land, there are many trails and trailheads that criss-cross the area. Many of these trails are popular with hikers and bikers alike seeking refuge from the urban landscape of the San Francisco Bay Area. Some of these trails, usually the mountaintop or ridgetop trails, provide spectacular views of the San Francisco Bay, Pacific Ocean, or both. Other trails, usually lower elevation, valley trails, lead into natural, dense groves of mature Douglas-fir and redwood trees, clear, open grassland, and shrublands. All roads leading to the many trailheads around Mount Tamalpais are usually open, but during fire season, some of these roads may be closed due to high fire risk.

The many roads, paved and unpaved, that cross the Mount Tamalpais region are particularly popular with mountain bikers, especially on weekends. Mount Tamalpais played a role in the birth of mountain biking many years ago. There has been considerable controversy over trail access for mountain bikes, both in terms of environmental impact and the safety of other trail users. As a result, bicycles have been banned from the majority of narrow, single-track trails, though bicycles are still allowed on fire roads. The fire roads on Mt. Tam are mixed-use and they are popular running, hiking, biking and horseback riding routes.[25]

Cookie Mueller, in her book Walking Through Clear Water In A Pool Painted Black, lists Mount Tamalpais as the location she goes with her friend Patrick to watch him perform a ceremony to evoke one of Beelzebub's footmen after collecting the necessary supplies from Anton LaVey, "America's foremost demonologist and devil worshiper of the moment".

^Billiter, Bill (January 1, 1985). "3,000-Year-Old Connection Claimed : Siberia Tie to California Tribes Cited". Los Angeles Times (Los Angeles). Archived from the original on 2014-11-28. Retrieved 2014-11-28. Another word with current Siberian translation, he said, is Tamalpais, the name of the mountain that is a landmark in the San Francisco Bay Area. "Pais" means place, he said, and "tamal" is a word used to describe one of the Siberian tribes.